US 2007/0161269 corresponding to JP-A-2006-303327 discloses an electronic circuit device configured as an electronic key transceiver. The electronic circuit device includes a circuit board having first and second sides opposite each other. Electronic elements are mounted on only the first side of the circuit board. The circuit board is encapsulated in a casing in such a manner that the second side of the circuit board is exposed to an outer surface of the casing. Thus, the second side of the circuit board defines part of the outer surface of the casing.
A method of making the electronic circuit device includes a placing process and an encapsulating process subsequent to the placing process. In the placing process, the circuit board is placed in a cavity of a mold (i.e., die) in such a manner that the second side of the circuit board can be held in close contact with an inner surface of the cavity. In the encapsulating process, a liquid resin material is injected under pressure into the cavity of the mold and then cured (i.e., solidified).
Since the circuit board is held in close contact with the inner surface of the cavity, the circuit board can be prevented from being greatly deformed by pressure and heat, which are applied to the circuit board in the encapsulating process.
In the method described above, after the resin material is injected into the cavity, air may be trapped in a gap between the electronic element and the circuit board. The air trapped in the gap may expand due to the pressure and heat and cause a bulge (bump or the like) on the second side of the circuit board. Since the second side of the circuit board is exposed to the outer surface of the casing, the bulge mars the appearance of the electronic key transceiver.